Draining the Land

Water that drains the land has a series of streams network which is filled from either the ground water or the water from the atmosphere, hydrologic cycle.

Forming Streams and Drainage Networks

Excess surface water (runoff) comes from rain, melting ice or snow, and ground water springs. On flat round, water accumulates in puddles ow swamps, but no slopes, it flows downslope in streams.

Where does the water in a stream come from? Recall that water enters the hydrologic cycle by evaporating from the Earth’s surface and rising into the atmosphere. After a relatively short residence time, atmospheric water condenses and falls back to the Earth’s surface as rain or snow that accumulates in various reservoirs. Some rain or snow remains on the land as surface water (in puddles, swamps, lakes, snowﬁelds, and glaciers), some ﬂows downslope as a thin ﬁlm called sheetwash, and some sinks into the ground, where it either becomes trapped in soil (as soil moisture) or descends below the water table to become groundwater. (the water table is the level below which groundwater ﬁlls all the pores and cracks in subsurface rock or sediment. Above the water table, air partially or entirely ﬁlls the pores and cracks.) Streams can receive input of water from all of these reservoirs (figure above). Speciﬁcally, gravity pulls surface water (including meltwater) downhill into stream channels, the pressure exerted by the weight of new rainfall squeezes existing soil moisture back out of the ground, and groundwater seeps out of the channel walls into the channel, if the ﬂoor of the channel lies below the water table.

Running water collects in stream channels, because a channel is lower than the surrounding area and gravity always moves material from higher to lower elevation. How does a stream channel form in the ﬁrst place? The process of channel formation begins when sheetwash starts ﬂowing downslope. Like any ﬂowing ﬂuid, sheetwash erodes its substrate (the material it ﬂows over). The efﬁciency of such erosion depends on the velocity of the ﬂow faster ﬂows erode more rapidly. In nature, the ground is not perfectly planar, not all substrate has the same resistance to erosion, and the amount of vegetation that covers and protects the ground varies with location. Thus, the velocity of sheetwash also varies with location. Where the ﬂow happens to be a bit faster, or the substrate is a little weaker, erosion scours (digs) a channel. Since the channel is lower than the surrounding ground, sheetwash in adjacent areas starts to head toward it. With time, the extra ﬂow deepens the channel relative to its surroundings, a process called downcutting, and a stream forms.

An example of headward erosion. The main stream ﬂows in a deep valley. Side streams are cutting into the bordering plateau.

As its ﬂow increases, a stream channel begins to lengthen at its origin, a process called headward erosion (figure above). Headward erosion occurs for two reasons. First, it happens when the surface ﬂow converging at the entrance to a channel has sufﬁcient erosive power to downcut. Second, it happens at locations where groundwater seeps out of the ground and enters the entrance to the stream channel. Such seepage, called “groundwater sapping,” gradually weakens and undermines the soil or rock just upstream of the channel’s endpoint until the material collapses into the channel; the collapsed debris eventually washes away during a ﬂood. Each increment of collapse makes the channel longer.

As downcutting deepens the main channel, the surrounding land surfaces start to slope toward the channel. Thus, new side channels, or tributaries, begin to form, and these ﬂow into the main channel. Eventually, an array of linked streams evolves, with the smaller tributaries ﬂowing into a trunk stream. The array of interconnecting streams together constitute the drainage network. Like transportation networks of roads, drainage networks of streams reach into all corners of a region, providing conduits for the removal of runoff.

Block diagrams illustrating five types of drainage networks.

The conﬁguration of tributaries and trunk streams deﬁnes the map pattern of a drainage network. This pattern depends on the shape of the landscape and the composition of the substrate. Geologists recognize several types of networks on the basis of the network’s map pattern (figure above).

Dendritic: When rivers flow over a fairly uniform substrate with a fairly uniform initial slope, they develop a dendritic network, which looks like the pattern of branches connecting to the trunk of a deciduous tree.

Radial: Drainage networks forming on the surface of a cone shaped mountain flow outward from the mountain peak, like spokes on a wheel. Such a pattern defines a radial network.

Rectangular: In places where a rectangular grid of fractures (vertical joints) breaks up the ground, channels form along the preexisting fractures, and streams join each other at right angles, creating a rectangular network.

Trellis: In places where a drainage network develops across a landscape of parallel valleys and ridges, major tributaries flow down a valley and join a trunk stream that cuts across the ridges. The resulting map pattern resembles a garden trellis, so the arrangement of streams constitutes a trellis network.

Parallel: On a uniform slope, several streams with parallel courses develop simultaneously. The group comprises a parallel network.

Drainage Basins and Divides

Drainage divides and basins.

A drainage network collects water from a broad region, variously called a drainage basin, catchment, or watershed, and feeds it into the trunk stream, which carries the water away. The highland, or ridge, that separates one watershed from another is a drainage divide (figure above a, b). A continental divide separates drainage that ﬂows into one ocean from drainage that ﬂows into another. For example, if you straddle the continental divide where it runs along the crest of the Rocky Mountains in the western United States, and pour a cup of water out of each hand, the water in one hand ﬂows to the Atlantic, and the water in the other ﬂows to the Paciﬁc. Three divides bound part of the Mississippi drainage basin, which drains the interior of the United States.

Streams That Last, Streams That Don’t

The contact between permanent and ephemeral streams.

Permanent streams ﬂow all year long, whereas ephemeral streams ﬂow only for part of the year. Some ephemeral streams ﬂow only for tens of minutes to a few hours, following a heavy rain. Most permanent streams exist where the ﬂoor (or bed) of the stream channel lies below the water table (figure above a). In these streams, which occur in humid or temperate climates, water comes not only from upstream or from surface runoff, but also from springs through which groundwater seeps. If the bed of a stream lies above the water table, then the stream can be permanent only when the rate at which water arrives from upstream exceeds the rate at which water inﬁltrates into the ground below. For example, the downstream portion of the Colorado River in the dry Sonoran Desert of Arizona ﬂows all year, because enough water enters it from the river’s wet headwaters upstream in Colorado; hardly any water enters the stream from the desert itself.

Streams that do not have a sufﬁcient upstream source, and whose beds lie above the water table, are ephemeral, because the water that ﬁlls a channel due to a heavy rain or a spring thaw eventually sinks into the ground and/or evaporates, and the stream dries up (figure above b). Streams whose watersheds lie entirely within an arid region tend to be ephemeral. The dry bed of an ephemeral stream is variously called a dry wash, an arroyo, or a wadi.